oojs-ui: Fix up addition to core

Follows-up I0736601e04a15c8.

* Move to its own directory instead of mixing in the oojs.js dir.
* This also fixes the jsduck errors as it was immediately being
  indexed with settings for oojs, eventhough oojs-ui correctly
  does not index without errors with core's jsduck config.
* Remove spurious .gitignore file.

Change-Id: I01f99deb3967d731d7b0e8bca2a2497b3c0151ac